1. Field of the Invention
The present invention relates to a process for producing a cubic system boron nitride from rhombohedral boron nitride.
2. Description of the Prior Art
Cubic system boron nitride has hardness comparable to diamond, and it is even superior to diamond in its properties as a grinding material for steels.
Heretofore, it has been common to use powder of hexagonal system boron nitride or its sintered product as the starting material for the production of a high pressure boron nitride. In a method for converting hexagonal system boron nitride to a high pressure boron nitride by subjecting the hexagonal system boron nitride to static high pressure or high pressure of a shock wave, the product thereby obtainable is composed mainly of wurtzite structure boron nitride with inclusion of a small amount of cubic boron nitride. The wurtzite structure boron nitride is inferior to cubic system boron nitride in its sintering property and hardness, and sharp corners are hardly obtainable by cleavage. Thus, it is inferior in its properties as grinding material.
In order to increase the yield of the cubic system boron nitride, it has been attempted to subject the hexagonal system boron nitride to a high temperature of at least 2,000.degree.K. under static high pressure, or to repeat the shock compression several times.
However, such methods require expensive apparatus and involve cumbersome operations, and yet the yield of the cubic system boron nitride is low. Further, it is also known to subject the hexagonal system boron nitride to a high temperature under high pressure in the presence of a catalyst. In this method, coarse cubic system boron nitride is obtainable. However, it is necessary to maintain the reaction conditions at a high temperature under high pressure for a long period of time, and accordingly, expensive equipments and operations are required.